dbcve.org
ZldOperating system · Zyxel

CVE-2023-35139

MEDIUM · 6.1 CVSS v3.1 Published 2023-11-28
Fix available
A fix is available. Upgrade to after 5.37 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
A cross-site scripting (XSS) vulnerability in the CGI program of the Zyxel ATP series firmware versions 5.10 through 5.37, USG FLEX series firmware versions 5.00 through 5.37, USG FLEX 50(W) series firmware versions 5.10 through 5.37, USG20(W)-VPN series firmware versions 5.10 through 5.37, and VPN series firmware versions 5.00 through 5.37, could allow an unauthenticated LAN-based attacker to store malicious scripts in a vulnerable device. A successful XSS attack could then result in the stored malicious scripts being executed to steal cookies when the user visits the specific CGI used for dumping ZTP logs.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

This is a stored cross-site scripting (XSS) vulnerability in the CGI program of multiple Zyxel firewall product lines (ATP, USG FLEX, USG20W-VPN, VPN series). An unauthenticated LAN-based attacker can inject malicious scripts into the CGI endpoint used for dumping ZTP (Zero Touch Provisioning) logs. When legitimate users subsequently access this CGI, the stored malicious scripts execute, allowing cookie theft.

MitigationApply available firmware updates from Zyxel that address this vulnerability. Until patches are applied, restrict LAN access to the affected CGI endpoint and monitor for suspicious activity.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
ZldOperating system
Affected:>= 5.10, <= 5.37>= 5.00, <= 5.37

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
Required
Scope
Changed
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify your Zyxel device model
    Log into the firewall administrative interface or check the device label to confirm the model (ATP, USG FLEX, USG20W-VPN, or VPN series).
    Affected if The model is one of ATP, USG FLEX, USG20W-VPN, or VPN series.
  2. Check the installed firmware version
    Access the firewall admin panel and navigate to System > Firmware or use the CLI command 'show version' to retrieve the Zld firmware version.
    Affected if The firmware version falls within the ranges Zld >= 5.10 and <= 5.37, or >= 5.00 and <= 5.37.
  3. Verify ZTP log CGI endpoint accessibility
    From a LAN-connected host, attempt to access the CGI endpoint used for ZTP log dumping (typically /ztp/cgi-bin/ or similar path on the firewall's web interface). Use a web browser or curl to check if the endpoint responds.
    Affected if The ZTP CGI endpoint is accessible from the LAN without authentication.
  4. Inspect for injected scripts in ZTP logs
    If you have administrative access, review the ZTP provisioning logs through the firewall admin panel or via CLI commands that dump ZTP-related logs. Look for any unexpected script tags, JavaScript code, or suspicious URL parameters.
    Affected if The ZTP logs contain encoded or obfuscated script tags, unusual HTML markup, or known malicious payload patterns.

You are affected if your Zyxel device is an ATP, USG FLEX, USG20W-VPN, or VPN series model running Zld firmware between versions 5.00 and 5.37, and the ZTP CGI endpoint is accessible from the LAN.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to a release after 5.37
Interim mitigation

Apply available firmware updates from Zyxel that address this vulnerability. Until patches are applied, restrict LAN access to the affected CGI endpoint and monitor for suspicious activity.

Recommended fix Moderate confidence

Firmware version 5.38 or later for all affected series (ATP, USG FLEX, USG FLEX 50W, USG20W-VPN, VPN)

  1. 1. Identify the specific Zyxel device model from the affected product list (ATP, USG FLEX, USG FLEX 50W, USG20W-VPN, or VPN series)
  2. 2. Navigate to the Zyxel support website (www.zyxel.com) and locate the firmware download section for your specific device model
  3. 3. Check available firmware versions; download and install firmware version 5.38 or later (the first version containing the security fix for this CVE)
  4. 4. Before upgrading, backup the current device configuration
  5. 5. Follow Zyxel's standard firmware upgrade procedure for your device series
  6. 6. After upgrade, verify the device is functioning normally and the CGI endpoint for ZTP logs has been patched
Caveat Review Zyxel release notes for version 5.38+ to check for any configuration or feature changes that may affect your deployment

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Zld Scoped from the published advisory
  • Consultation3.0 h
  • Implementation4.0 h
  • Testing3.0 h
  • Review / QA2.0 h
12.0 hours of engineering $2,130
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2023-35139 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data